1. Field of the Invention
The present invention relates to lighting units for use in inspecting products for their outward appearances, flaws or finished conditions, or in position detection or like applications.
2. Description of the Related Art
Hitherto, halogen lamps are known as representatives of light sources for use in lighting units of this type. Since such a halogen lamp has limited freedom in selection of a suitable place to which the halogen lamp is to be installed due to its bulkiness and since it is difficult to condense light from the halogen lamp, it has been a conventional practice to illuminate a work not directly from a halogen lamp but indirectly by guiding light from the halogen lamp through an optical fiber to a head attached to the front end of the optical fiber, as described in Japanese Patent Laid-Open Gazette No. HEI 5-248820. Such an optical fiber is used because it has flexibility and can be cut to a desired length and hence allows the halogen lamp to be freely located at an easy-to-install place even if the place is, for example, remote from the lighting site.
A lighting unit for illuminating works that are constantly at non-fixed positions such as those inaccurately positioned on and fed one after another by a conveyor unit, needs to have a function allowing the head having an illuminating aperture to move frequently in accordance with the position of each work.
Of course, the optical fiber used in such a lighting unit moves with frequent movements of the illuminating aperture. Since such an optical fiber has flexibility, it has heretofore been considered that such a lighting unit can accommodate well to applications where the head needs to move as described above.
Actually, however, particularly where a relatively long (for example 2 or 3 meters or longer) optical fiber is used, it is likely that an up-sized driving mechanism is needed for moving the head while causing the associated optical fiber to accompany the head or that movement of or position control over the head becomes difficult, because generally an optical fiber is relatively bulky and heavy as compared to electric wires or the like.
Further, since optical fibers are less flexible than electric wires, they are likely to be damaged in a relatively short time by frequent bending and moving, which may result in problems in respect of the reliability, lifetime and the like of the lighting unit.
It is possible to use an LED, which recently has been attracting attention as a substitute for a halogen lamp, as a light source. However, where a multiplicity of LEDs are directly fitted to a head without using an optical fiber, a problems arises that downsizing of the head and light condensing are difficult. For example, when a very small object, such as a component mounted on a printed circuit board, is to be illuminated, such a head fitted directly with LEDs has a relatively large minimum focal diameter and hence illuminates the object as well as unwanted portions, which results in inefficient lighting.
The present invention, which is not made until the conventional concept of moving the head by utilizing the flexibility and the length adjustability of an optical fiber has been abandoned completely, intends to solve the foregoing problems at a time by taking advantages of the lightness, compactness and the like of an LED light source device as well as the advantage of an optical fiber head in its possibility to reduce the size thereof.